Abstract
We reassess the absolute and relative sea level changes at 38 tide gauge stations in the earthquake-affected Western North Pacific for the 1993–2015 period, focusing on the vertical land motion (VLM) which is crucial for narrowing the gap between these estimates. In this area, simply discarding all earthquake-affected sites, one overestimates the average regional sea level rise by more than 0.5 mm/year. Disregarding VLM would lead to misestimating local sea level trends between 0.2 and 7.6 mm/year. If accounted for, but modeled as linear during the entire time span, VLM leads to regional absolute sea level rise errors of up to 0.4 mm/year. Therefore, we introduce a new methodology that better represents the Global Positioning System (GPS)-derived nonlinear VLM by accounting for co-seismic offsets, changes in the vertical velocities and post-seismic transient. Also, for the first time, a combination of white and power-law noises is added to this nonlinear model to derive proper uncertainties of VLM. We find a maximum difference of 15.3 mm/year between pre- and post-seismic vertical velocities. The GPS-sensed vertical co-seismic displacement approaches 36 mm. Assuming the changes in vertical velocities and displacement due to the tectonic movements is not accounted for, and then, estimating absolute sea level rise from tide gauges can result in an error of 10 mm/year. Introducing a new nonlinear VLM model improves absolute tide gauge sea level estimates by 20% on average. Finally, for the reconstructed Western North Pacific sea level, altimetry agrees best with tide gauge data corrected employing the new nonlinear VLM model.
Highlights
Estimates of relative sea level rise derived from tide gauge records are affected both by vertical land motion (VLM) and by absolute sea level rise due to ocean warming, salinity change, freshwater influx and other factors (WӧppelmannElectronic supplementary material The online version of this article contains supplementary material, which is available to authorized users.1 3 Vol.:(0123456789) 102 Page 2 of 12GPS Solutions (2019) 23:102 opportunity is provided nowadays by precise Global Positioning System (GPS) observations recorded at thousands of permanent stations around the globe
For the Western North Pacific and other earthquake-affected areas such as Alaska, Indonesia or South America, the collisions of tectonic plates and associated motion play the main role in relative sea level rise and represent the largest contributor to VLM
Relative sea level trends estimated for tide gauges do not differ significantly from the altimetry estimates
Summary
Estimates of relative sea level rise derived from tide gauge records are affected both by vertical land motion (VLM) and by absolute sea level rise due to ocean warming, salinity change, freshwater influx and other factors We argue that changes in vertical velocities over the years, small co-seismic offsets and post-seismic transient, that did not lead to the removal of time series from sea level reconstruction, may have been mis-modeled Accounting for these effects, we introduce a completely new “optimized” VLM model for the Western North Pacific basin affected by the megathrust Tohoku-Oki earthquake (a magnitude 9.1 Mw, National Earthquake Information Center, NEIC) on March 11, 2011, with epicenter at 38.322°N, 142.369°E. We describe the results in terms of VLM, relative and absolute sea level rises, proving that the nonlinear model we introduce fits the GPS-derived position time series better than the commonly used linear model and narrows the gap between altimetry and tide gauge estimates. The time span of our analysis is selected from January 1993 to December 2015, which overlaps with available of altimetry records and GPS data
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